00193407-02 HWWS SIPLACE Machine Accuracy EN.pdf - 第7页
© Siemens AG . . Änderungen vorbehalten SIPLACE Placement Systems h ow do we define the capability indices? ¾ is it an approximate or exact estimatio n of t he number of defects??? ¾ is it a comparison of specifications?…
© Siemens AG . . Änderungen vorbehalten
SIPLACE
Placement Systems
Capability Indices
Indices are used to assess the quality of machines, steps in a process and complete processes.
A single index does not provide sufficient information for the complete characterization of a
process. A comprehensive overview of the qualification status of the process under examination
can only be achieved by studying a combination of several indices.
Placement accuracy takes the deviation from the actual position and the target position of the
components into account.
This contains positioning errors for the linear axis, rotational errors at the placement head,
twisting of the gantry system, errors at the vision modules, .....
More and more fine pitch components are being placed and an increasing number of very small
passive components, size 0402, 0201 and more smaller. Consequently, higher and higher
placement accuracy is a demand of placement machines.
Many natural and technical processes which occur randomly (stochastically) can be described by
a normal distribution.
The deviation of the placement position from the target value consists of a reproducible
component and a random component. The reproducible component can be kept relatively low
through calibration measures. The random component is close to normal distribution (this has
been experimentally confirmed). The normal distribution provides an exact mathematical model
for describing placement accuracy.
Machine Capability
© Siemens AG . . Änderungen vorbehalten
SIPLACE
Placement Systems
how do we define the capability indices?
¾ is it an approximate or exact estimation of the number of defects???
¾ is it a comparison of specifications??
¾ is it a question of interpretation?
example pick & place head Siplace 80F
sigma Cm; Cmk accuracy defect rate
±
1
68,27% 0,33
±
12,5
μ
317300
±
2
95,45% 0,66
±
25
μ
45500
±
3
99,73% 1
±
37,5
μ
2700
±
4
99,994% 1,33
±
50
μ
60
±
5
99,99994% 1,66
±
62,5
μ
0,6
±
6
99,9999998% 2,00
±
75
μ
0,002
which one is better??
50 µm / 4 sigma ⇔ 60µm / 5 sigma
it‘s not visible at the first view !!
Machine Capability
© Siemens AG . . Änderungen vorbehalten
SIPLACE
Placement Systems
Placement Accuracy
What does a statement of placement accuracy +- 50 um at 4 sigma mean?
The 4 sigma limit value of the normally distributed deviation of the placement position is 50 um.
This enables the user to rate the reliability of the placement process.
The defect probability at 4 sigma is 60 ppm. The user can assume that placement error exceeds
50 um for only 60 out of one million placed components.
Limit value: 37.5 um (3 sigma) 2600 ppm
50,0 um (4 sigma) 60 ppm
75,0 um (6 sigma) 0.002 ppm
During this process, it becomes obvious that identical placement accuracy can be described in
different ways. The statements 50 um at 4 sigma, 25 um at 2 sigma and 75 um at 6 sigma are
equivalent, the standard distribution = 12.5 um.
Factors influencing placement accuracy:
¾ placement machine
¾ tolerances and twist of the PCB
¾ dimensional accuracy of the components
¾ bending of component pins
¾ ..........................
Machine Capability